Three-act structure really lends itself particularly well to convention games, where there's a limited time frame in which to play through a given story/dungeon/scenario. I find that pretty much every game that I run at a major convention--which I've been doing for at least 10 years, plus my own home games--uses some variation on a three-act structure.

That said, there is a *lot* that a good GM can learn from alternate methods of narrative structure. I'm actually working on a series of essays designed to analyze this, focusing primarily on non-linear storytelling (such as that found in movies like Memento, Snatch, and Lock, Stock and Two Smoking Barrels).

I remember covering this, albeit in a far more simplified way, at the tender age of 7 at school (Primary 3), where our teacher called it the "Beefburger":

The top of the bun is fairly plain but has the seeds, hinting at whats to come.

The middle is the delicious part, with all the action.

The bottom of the bun is bringing us back down from our meat-frenzy and brings us to a conclusion.

Obviously this version falls apart under the scrutiny but I was only 7 or so. Over the years though Im aware I've drifted from that method, opting to create rolling narratives which tend to segway into future campaigns rather than providing a clear conclusion.

Interestingly however, GMing Firefly has made me think a lot more about narrative structure as that clearly requires scene definitions and "episode" structures.